Coatings based on epoxy resins are important industrial products. The largest volume of these products is used for the protection and decoration of large metal or concrete structures such as bridges, ships, industrial tanks, etc., where application of the coating must be performed under ambient conditions. Epoxy resin coatings provide a combination of corrosion resistance, water resistance, abrasion resistance and solvent resistance.
Most epoxy resin coatings designed for ambient application employ polyfunctional amines as the curing agent, either alone or in some cases in combination with other curing agents. Several classes of amine curing agents are used commercially, including low molecular weight diamines such as metaxylene diamine, isophorone diamine, polyether diamines, and polyamides (usually the reaction products of dimer fatty acids (such as PRIPOL 1017 and PRIPOL 1048 (trade marks, ex Uniqema)) and polyamines). They are described in W. R. Ashcroft, Curing Agents for Epoxy Resins, in B. Ellis (ed.), Chemistry and Technology of Epoxy Resins, Blackie Academic and Professional, London, 1993, pp. 37-71.
However, there are a number of problems associated with existing epoxy resin curing agents. Low molecular weight diamines for example are often volatile and sometimes toxic, and may result in an epoxy resin having insufficient flexibility.
Polyamides, on the other hand, can have relatively low reactivity, particularly at temperatures close to zero, and also generally have high viscosity which means that they must be used with organic solvents. For both environment and safety reasons there is a general requirement to reduce solvent levels in coatings, including epoxy coatings. Thus, there is a need for improved low viscosity curing agents for epoxy resins.
Restrictions on the use of solvents may require that low molecular weight epoxy resins be used in place of the preferred, higher molecular weight epoxy resins irrespective of the viscosity of the curing agent employed. The use of low molecular weight epoxy resins increases the drying time of the coating. Thus, there is a need for curing agents that reduce the drying time of epoxy coatings, particularly those based on liquid epoxy resins.
For good protection of substrates, such as concrete and metal, it is necessary that the coating maintain good adhesion to the substrate, particularly under wet conditions. While epoxy coatings generally have good adhesion, there is still a need for improved adhesion, particularly on substrates such as cold-rolled steel (CRS).
Finally, there is a need for curing agents that can lead to epoxy coating compositions with greater corrosion resistance, leading to coatings with longer life in service.